The 8 th International Workshop on the Physics of Compressible - PDF document

The 8 th International Workshop on the Physics of Compressible Turbulent Mixing Experimental study into Rayleigh-Taylor turbulent mixing zone heterogeneous structure Yu. A. Kucherenko, A. P. Pylaev, V. D. Murzakov, A. V. Belomestnih, V. N. Popov, A. A. Tyaktev Academician E. I. Zababakhin Russian Federal Nuclear Center – All-Russian Research Institute of Technical Physics P. O. 245, Snezhinsk, Chelyabinsk region, 456770, Russia E-mail: kucherenko@five.ch70.chel.su Abstract: The heterogeneous structure study has been performed by means of a “light-sheet” technique at the SOM gas-dynamic accelerator. The investigated system consisted of three layers of different density liquids. For leading out the information from the mixing zone inner region illuminated by the “light-sheet”, visualizing particles were seeded into one of the liquids. The visualizing particles, which got into the “light-sheet”, diffused light, and at the same time photo images of the liquid fragments, contained the visualizing particles, were formed by a light-sensitive receiver. For the error reduction, refractive indexes of all the three liquids were equalized. A special test has been conducted for determining of measurements inaccuracy. Experiments have been performed for two values of acceleration of artificial field of gravity. Distributions of liquid fragments sizes are showed in the form of bar charts for different moments of time. 1 Introduction Up to the present, the gravitational turbulent mixing heterogeneous structure is insufficiently known though it has been made considerable efforts for solution of this problem. There had been made an attempt to value fragments scales of different density miscible liquids at their gravitational mixing by an electro contact technique in experimental work [1]. In this work, a qualitative result had been obtained. According to this result, mixing at the unstable stage occurred by large fragments but at the turbulent mixing stage fragment sizes amounted ∼ 1 mm. In work [2] structure of the Richtmyer-Meshkov turbulent mixing of different density gases had been studied by a “laser knife” technique. In this work, there had been obtained photo images of non-uniformities in inner sections -of the mixing zone that gave an idea of the gases mixing character. In experimental & numerical works [3,4] the structure of gravitational turbulent mixing of miscible liquids with educing of molecular component had been studied for low Atwood numbers. Molecular part evaluations of mixing and density fluctuations were obtained in these works. In experimental & numerical work [5] fractal dimension evaluations of constant concentration contours had been obtained for the Rayleigh-Taylor turbulent mixing of liquids for low Atwood numbers. In work [6] density profiles of mixing liquids had been obtained from photo images of mixing zone sections by a “laser sheet” technique. In the present work, an attempt of direct determination of immiscible liquids fragments sizes at their Rayleigh-Taylor turbulent mixing has been made. A “light sheet” technique has been employed for this study. It is known that for immiscible liquids the smallest size of fluid elements, which are in result of fragmentation, depends on relation between inertial forces determining by acceleration of a system and resistant forces determining by surface tension of given couple of liquids (Kolmogorov’s criterion). A. V. Polionov offered the following relation for evaluation of the minimum size of fluid elements:   4 / 7 σ ρ / 1   ≈ d 4 , 3 . (1)   A g L 1 / 7   1  RFNC-VNIITF, 2001

8 th IWPCTM 2 ρ − ρ Here σ σ is surface tension value, ρ is density, = A 2 1 is Atwood number, g 1 is acceleration of ρ + ρ 2 1 artificial field of gravity, L - is turbulent mixing zone size. So the minimum size of fluid elements for chosen experimental system containing immiscible liquids depends on acceleration value g 1 . Therefore experiments have been performed for two essentially different accelerations. 2 Experimental technique Experiments were performed at the SOM installation described in work [7]. The measuring module of the installation represents a vertical channel, in upper part of which an ampoule containing studied liquids is placed at initial moment of time. The ampoule is accelerated by a gas flow, and a liquid system placed inside of the ampoule becomes unstable because of the acceleration is directed from a heavy liquid to a light one in the coordinate system connected with the ampoule. Owing to unstable a turbulent mixing arises at the contact boundary of the liquids. In the present work the measuring module was equipped with 14 horizontal light channels located with a step of 56 mm. Each channel contained the “light sheet»-forming block. The sketch of a horizontal section of the light channel is shown in Fig. 1. Fig. 1. Sketch of a horizontal section of the light channel Light radiated by a pair of impulse sources (2), which is located in case (1), transforms by means of cylindrical optics (3) and diaphragms (4) to a luminous flux having a form of “light sheet” of thickness ∼ 1.5 mm. The “light sheet” comes into the ampoule (5) from two sides and illuminates chosen section of the mixing zone. Scheme with two-side coming of the “light sheet” is chosen from consideration of uniform illumination of chosen section along the ampoule length. Visualizing particles inserts into one of the liquids. Light scattered by the visualizing particles, which are in the “light sheet” section, finds itself in the photo recorder (7) where a photo image of the mixing zone section forms. This photo image is some set of fragments of that liquid which contains visualizing particles. 3 Sensitivity of the technique The light channel sensitivity, i. e. the least registered size of non-uniformities, depends on a set of factors, so that it was determined by the most direct method – with using some models. There

8 th IWPCTM 3 used jets specified size and form as models of non-uniformities. The jets formed by special formers two of which is shown in Fig. 2. Fig.2. Formers for jets forming The formers were located inside the ampoule at the contact boundary of the liquids which were aqueous solution of glycerin and benzine with the density ratio n = 1.6. Visualizing particles were in the aqueous solution of glycerin. At moving a former down, the heavy liquid containing visualizing particles passes through the holes producing jets, form and diameter of which corresponds to the form and diameter of the holes in the former. The photo recorder only takes those images for which jets find themselves in the “light sheet” section. Photo images of the jets are a , there are distinctly seen four jets of diameter 5 mm found themselves in shown in Fig. 3. In Fig. 3 the “light sheet” section and not seen other jets not found themselves there. In Fig.3 b , there are a . In distinctly seen seven jets of diameter 3 mm formed with applying the former shown in Fig. 2 c and Fig. 3 d , there are seen by four jets of diameter correspondingly 2 mm and 1 mm. These Fig. 3 b . Holes of that former were placed on an angle to jets were formed with the former shown in Fig. 2 the “light sheet”. It is seen that the images only correspond to those jets which found themselves in the “light sheet” section. Jets of diameter less than 1 mm do not practically have images. Obtained results give a possibility to assert that: 1. Concentration of visualizing substance is enough for sharp image acquisition of non- uniformities of sizes not less than 1 mm; 2. Those non-uniformities, which do not find themselves in the “light sheet” section, do not have photo images. Fig. 3. Photo images of jets taken with special formers